Field sampling and processing
For each tree species, four healthy individuals with a similar diameter
at breast height were randomly selected. To ensure sampling
independence, the selected individuals were at least 10 m apart. We
sampled the green and senesced foliage of these trees in autumn 2018.
Specifically, the fully mature and expanded foliage from the most distal
branches of each tree was cut from the sun-exposed canopy using a tree
trimmer and bamboo extensions. Green foliage with visible biological or
physical damage was excluded. More than 60 green leaves from each
species were selected for further analysis. Thirty leaves per tree were
placed in a valve bag and stored at 4 °C to measure morphological
traits. The remaining samples were placed in envelopes to measure
chemical traits. Due to the tree height and sampling difficulties, we
spent two weeks per month during the whole autumn collecting senesced
foliage. The senesced foliage was sampled directly beneath the focal
trees from the recently fallen fresh litter on the ground (Huang et al.
2007). The senesced foliage with no visible degradation was well mixed
for each species. The first set of subsamples was air-dried for chemical
analyses, and the second set of subsamples was used for litter
decomposition experiments.
Belowground samples were collected when sampling aboveground foliage.
Four 1 × 1 m plots were identified within a 2 m distance from the tree
stem in four directions from each target tree. The surface mineral soils
(0–20 cm) at the plots were loosened with a pickaxe and spade, and
sieved (2-mm mesh size) to sample the initial materials. Meanwhile,
intact roots (more than five root orders) were cut from the main lateral
roots, following the procedure described by Guo et al. (2008). Each root
sample was divided into two subsamples for different analyses. One
subsample was gently washed in
deionised water to remove adhering
organic matter and immediately fixed in formalin-aceto-alcohol solution
(90 ml 50% ethanol, 5 ml 100% glacial acetic acid, and 5 ml 37%
methanol) to measure anatomical traits. The other subsample was placed
into valve bags, incubated with ice bags, and stored at -20 °C for later
dissection and analysis of morphological and chemical traits (Guo et al.
2008).